502 STRUCTURE OF VERTEBRATA. 
seems not unlikely that the alimentary tract was originally a 
ciliated tube. 
At the posterior end an ectodermic invagination or proctodzeum meets 
the closed archenteron, and at the junction the two epithelial layers 
give way, so that an open tube is formed. 
The formation of the anus does not take place close to the posterior 
end of the primitive gut, but at a point some short distance iri front of 
this. In consequence the so-called post-anal gut is formed. This is 
continuous with the neurenteric canal, and so communicates with the 
neural canal. The post-anal gut attains in Elasmobranchs a relatively 
considerable length. It has been very frequently found in Vertebrates, 
and is probably of universal occurrence. After a longer or shorter 
period it becomes completely atrophied, and with it the communication 
between neural and alimentary canals is completely destroyed. In 
some Fishes and Amphibians the anus is formed directly from the 
blastopore. om 
Speculative.—The primitive gut was probably a smooth straight 
tube, but the rapid multiplication of well-nourished cells would tend 
to its increase in diameter and in length. But on increase in both 
directions the slower growth of the general body would impose limita- 
tions, and in this we may find the immediate growth-condition deter- 
mining the origin of folds, crypts, czeca, and coils, which would be 
justified by the increase of absorptive and digestive surface. There 
are regular longitudinal folds in AZyxzne, cross-folds traversing these 
would form crypts, which may be exaggerated into the pyloric caeca of 
Teleosteans and Ganoids, while other modifications would give rise to: 
‘*spiral valves” and the like. In the same way it may be suggested 
that the numerous important outgrowths of the mid-gut, such as lungs, 
iver, pancreas, and allantois, so thoroughly justified by their usefulness, 
may at first have been due to necessary conditions of growth—to the 
high nutrition, rapid growth, and rapid multiplication of the endoderm. 
It may be noted that in the development of the Amphibian Mecdurus, 
there are hints of more numerous endodermic diverticula (Platt). It is. 
also said that the hypochorda—a transitory structure—arising below and 
subsequent to the notochord, is in part due to a series of dorsal out- 
growths from the gut (Stdhr). Even the notochord, which arises as. 
a median dorsal fold, may be speculatively compared to a typhlosole— 
folded outwards instead of inwards. The future elaboration of the 
organs which arise as outgrowths of the gut would, however, 
depend on many factors, such as their correlation with other parts. 
of the body, and would at each step be affected as usual by natural 
selection. 
[TaBLE. 
